This invention relates to failure detection devices for convertors.
Thyristor convertors which convert alternating current into direct current comprise a plurality of series connected thyristors and fuses connected between an alternating current voltage source and a direct current load. For the purpose of detecting the occurrence of failures in these thyristor convertors, blown fuse detectors which detect the blowing out of the fuses are usually provided. Thus, when one of the thyristors is subjected to abnormal conduction failure, i.e. becomes abnormally conductive in the reverse direction, a short circuit occurs through such thyristor, and the fuse that is series connected therewith is blown out, which in turn is detected by the blown fuse detector. But in the case of abnormal nonconduction failure, i.e. when one of the thyristors becomes abnormally nonconductive in the forward direction, because of, for example, a failure in the firing circuit of the thyristor, the failure does not usually lead to the blowing out of the series connected fuse, and thus may not be detected by the blown fuse detectors.
It has been proposed to detect such abnormal nonconduction failure of the thyristors by sensing the periods in which the thyristors are conductive. Such conductive period sensor, however, must be provided for each thyristor, and must be electrically insulated from the main convertor circuit, which results in complicated and expensive failure detection circuits.